Similar to digital electronics, digital photonic circuits/chips can be built using logic gates, in which light signals, instead of electrical ones, are used to drive the device. When using light, we have two dimensions available for information encoding and computing, which are the light intensity and the wavelength. In other words, we can use N (N≧1) number of lightwaves, and each one can be at either zero intensity, to represent the binary 0's, or a pre-selected high intensity, to represent the binary 1's. For computing purposes, we can use either the intensities to represent information or the wavelengths or both. If the intensity is used for information encoding, one can have a multiple binary system with each lightwave having two intensities, one for 0's and the other for 1's. If only one wavelength is used in this case, it becomes a single binary system. On the other hand, if the wavelengths are used to encode information, in this case N≧2, we can have an N-valued system with each lightwave being at a constant intensity to be one of the values in the N-valued digital system. If two wavelengths are used in this case, it also becomes a binary system, with one wavelength representing the 0's and the other representing the 1's. However, if both the intensities and the wavelengths are used for information representation and manipulation, we can have a two dimensional logic system that not only provides high computation capacity but also can be constructed in a way so that the transitional functions among the digital values in the system to be simple and implantable by optical semiconductors. Based on these considerations, we constructed an N-valued digital logic system that defines the transition functions among the N values to achieve N-nary computing using both light intensities (only two intensities are used) and wavelengths (N number of wavelengths with N≧1). An N-nary Digital Photonic (NDP) system is established by implementing the transition functions through N-nary phonic logic gates. Using the photonic logic gates one can construct N-nary digital logic photonic circuits/devices. This system was disclosed in U.S. Pat. No. 6,778,303. An N-nary Optical Random Access Memories (O-RAM), according to the N-valued digital logic, was disclosed in U.S. Pat. No. 6,647,163. The design processes of any digital optical devices using the logic gates are similar to those of the binary Boolean digital system. But, the logic used is the N-valued digital logic instead of the Boolean binary logic.
Song (U.S. Pat. No. 6,647,163) constructed N-nary optical logic gates using Semiconductor Optical Amplifiers (SOAs). While the logic gates constructed by SOAs can produce the logic operations, they are not ideal in terms of performances including operation speed and power consumptions, as SOAs were originally designed for optical signal amplifications rather than signal switching. Furthermore, the sizes of SOAs are also relatively large in the realm of digital circuitry, which prevents large scale circuits from being integrated into a semiconductor chip. Another approach of implementing the N-nary photonic logic gates is to first design the photonic transistor (PT), which is a micro-device that allows one lightwave to switch on or off another lightwave. The transistors are then used to construct the photonic gates. Although the structure of the PT can be similar to that of a SOA, it is optimized for optical signal switching rather than amplification. In particular, the size of the PT can be much smaller than that of a SOA, as signal amplification is not required in a photonic transistor, which is very important for digital systems as it allows for a large number of transistors to be integrated into one chip for complex circuits. Smaller size optical semiconductor structure can also significantly reduce the switching time, or in other words increasing the operation speed, and reduce power consumption of the logical gates which are two additional important benefits in practical applications.
This present invention is an N-nary photonic transistor based on a heterojunction optical semiconductor microstructure which is similar to that of a SOA but with different dimension proportions in the structure and most importantly a much smaller size. This present invention of the PT makes it possible to integrate a large number of logic gates into one optical semiconductor chip for practical applications. In the meantime, the current PT also enables much faster logic gates and much lower power consumptions.
The photonic transistor design and computer simulation results are presented. The photonic transistor design presented here has the features of being small in size and power consumption, which makes it useful for integrated photonic chips. An example of using the photonic transistor to construct an optical AND (O-AND) gate (Refer to U.S. Pat. No. 6,778,303) is also presented.